1. Field of the Invention
The present invention relates to an illumination device for use in a transmissive liquid crystal display or the like.
2. Description of the Prior Art
FIG. 11 is a longitudinal sectional view showing the structure of a liquid crystal display equipped with a conventional illumination device of an edge-light type. This liquid crystal display is provided with an illumination device 1 and a liquid crystal display panel 2 illuminated by the illumination device 1. The illumination device 1 is provided with a main body 3 including tubular light sources 8, a frame 4 fitted to the main body 3, and a plurality of sheets 5, 6, and 7 arranged between the main body 3 and the frame 4.
The main body 3 is provided with a box-shaped lamp case 9 open at the top, a reflecting plate 10 arranged inside the lamp case 9 and shaped like a flat tube, and a light guide plate 11 arranged inside the reflecting plate 10 and having a diffusive pattern formed or printed on its bottom surface. The light sources 8 extend in the direction perpendicular to the plane of the figure, and are supported and fixed, through unillustrated supporting members, in spaces formed between each side surface of the light guide plate 11 and the side wall of the reflecting plate 10 facing it so as to be placed in the vicinity of the two side surfaces (light entrance surfaces) of the light guide plate 11.
The reflecting plate 10 is so formed as to extend parallel to and close to the inner surfaces of the bottom and side walls of the lamp case 9 and on the bottom surface of the frame 4, and has a part of its top wall cut out along the axial direction to form an opening. The top wall of the reflecting plate 10 covers the light sources 8 from above, and reaches both side edges of the top surface of the light guide plate 11.
The sheets 5 to 7 have diffusive, condensing, or other optical properties, and are all so formed as to transmit light. The sheets 5 and 7 are diffusive sheets, and the sheet 6 is a condensing sheet. The sheets 5 to 7 are so arranged as to face each other, and are supported and fixed on the main body 3 by being sandwiched, at their edges, between the frame 4 and the reflecting plate 10 so as to face an opening 4a of the frame 4.
The liquid crystal display panel 2 is arranged above the frame 4 so as to face the opening 4a of the frame 4. The placement of the sheets 5 to 7 between the frame 4 and the reflecting plate 10 permits the liquid crystal display panel 2 to be illuminated with uniform light all over its surface.
FIG. 12 is a longitudinal sectional view showing the structure of a liquid crystal display equipped with a conventional direct backlight, which illuminates a display panel directly from beneath. Here, such components as have their counterparts in the liquid crystal display shown in FIG. 11 are identified with the same reference numerals, and overlapping descriptions will not be repeated. In this liquid crystal display, the main body 3 is provided with a reflecting plate 12 shaped like a bottomed flat tube, and a diffusive plate 13 that is arranged on the top surface of a flange 12a formed around the opening of the reflecting plate 12 and that has a diffusive pattern printed on its top surface.
The light sources 8 extend in the direction perpendicular to the plane of the figure, and are supported and fixed, through unillustrated supporting members, inside the reflecting plate 12. The sheets 5 to 7 are supported and fixed on the main body 3 by being sandwiched, at their edges, between the frame 4 and the diffusive plate 13 so as to face the opening 4a of the frame 4.
In the conventional illumination devices described above, when the sheets 5 to 7 are placed between the frame 4 and the reflecting plate 10 (or the diffusive plate 13), the sheets leave a clearance C of 0 to 0.1 mm in the direction of their thickness. The clearance C here is the value given by Lxe2x88x92T, where L represents the length, as measured in the direction of the thickness of the sheets 5 to 7, of the space (hereinafter called the sheet placement space) in which the sheets 5 to 7 are sandwiched between the frame 4 and the reflecting plate 10 (or the diffusive plate 13), and T represents the sum of the thickness of the sheets 5 to 7.
When the sheets having optical properties as described above (hereinafter called the optical sheets) exhibit a high degree of thermal expansion/contraction and their bases are made of materials prone to deformation due to thermal stress, under conditions where the ambient temperature around the liquid crystal display exceeds 85xc2x0 C. as when, for example, the liquid crystal display is employed in car-mounted equipment mounted on a car under the flaming sun in summer, the optical sheets 5 to 7 curl within the space between the frame 4 and the reflecting plate 10 (or the diffusive plate 13), as shown in FIGS. 13 and 14, under the influence of heat. This causes the sheets 5 to 7 to interfere with each other and develop wavelike bends, and thus leads to degradation in the display quality of the liquid crystal display. Specifically, by bringing the liquid crystal display into a display state and observing it obliquely from above, from below, from the left, or from the right, one can visually recognize the bends in the optical sheets 5 to 7.
In tropical or desert regions of the world, it is not rare that the ambient temperature around the liquid crystal display exceeds 95xc2x0 C. Even in such a harsh temperature environment, the liquid crystal display needs to maintain satisfactory display quality.
Incidentally, as shown in FIG. 15, an optical sheet is given its function by forming on the surface of a base 21 made of polyethylene terephthalate (hereinafter abbreviated to PET) a coating layer 22 that is a thin film of a material having a condensing, diffusive, or other optical property.
In a liquid crystal display, in addition to an optical sheet, a shielding sheet is also used as a measure against unwanted emission such as electromagnetic radiation and noise. A shielding sheet is given its function by forming on the surface of a base 21 made of PET a coating layer 23 that is a thin film of a transparent conductive material. A shielding sheet is a transparent sheet.
These sheets are formed by being punched out from a large master sheet into the desired shapes. The sheets are thin plates of resin, and are thus relatively soft. Therefore, unlike metal sheets, of which the edges are liable to injure the hands of their handlers during assembly and transportation, there is no need to treat the edges of the sheets so as not to injure their handlers"" hands; that is, the sheets are used as they are punched out.
In these sheets, the base 21 has a different thermal expansion speed from the coating layer 22 or 23, and therefore, as temperature rises, the sheets curl. The greater the rise xcex94T1 in temperature, the more conspicuous the phenomenon. Likewise, the base 21 has a different thermal contraction speed from the coating layer 22 or 23, and therefore, as temperature falls by xcex94T2 from a high temperature to an ordinary temperature, the sheets develop conspicuous curl even during cooling.
For example, in a diffusive sheet having properties as described above, when thermal expansion/contraction produces a force that tends to curl the sheet, the sheet 14 develops, as shown in FIG. 16, wavelike bends where there is a scant margin in the direction of the thickness of the sheet 14 as between the frame 4 and the reflecting plate 10, i.e. where there is nowhere for stress to escape.
Such bends can be reduced by, as shown in FIGS. 17A and 17B, bonding the sheet 14, along all four sides thereof, to the inner surface of the frame 4. This helps prevent bends around the bonded portion 15 of the sheet 14, but instead causes large bends in the central portion of the sheet 14. Thus, by bringing the liquid crystal display into a display state and observing it obliquely from above, from below, from the left, or from the right, one can visually recognize the bends in the sheet 14.
Such bends can be reduced by, as shown in FIGS. 18A and 18B, bonding the sheet 14, along only one side thereof, to the inner surface of the frame 4. This causes concentration of stress on the opposite side to the bonded portion 15 of the sheet 14, and thus cause large bends on the opposite side to the bonded portion 15 of the sheet 14. Thus, by bringing the liquid crystal display into a display state and observing it obliquely from above, from below, from the left, or from the right, one can visually recognize the bends in the sheet 14.
Although diffusive sheets are dealt with in FIGS. 16, 17A, 17B, 18A, and 18B, the same phenomenon occurs also with optical sheets of other types and with shielding sheets. Moreover, this phenomenon occurs not only in the case of an illumination device of an edge-light type but also in the case of a direct backlight.
The applicant of the present invention once proposed, in Japanese Patent Application Laid-Open No. H11-306835, a technique that exemplifies a measure to overcome the problem described above. According to this technique, a chassis provided near an optical sheet so as to hold the edges of the optical sheet is so shaped as to hold the optical sheet at intervals (i.e. at discreet spots). Alternatively, another member, separate from a chassis and formed as a sheet-shaped or plate-shaped member, is provided between the chassis and an optical sheet so as to hold the optical sheet at intervals. In either way, wavelike bends that develop in the optical sheet are passed along toward four sides of the display surface of the liquid crystal display so that there remain no bends in the central portion of the display surface, and simultaneously the bends in the peripheral portion of the display surface are alleviated.
However, this technique does not provide a sufficiently effective measure against the problem under conditions where the ambient temperature around the liquid crystal display exceeds 85xc2x0 C.
Quite imaginably, also in illumination devices of older designs for use in liquid crystal displays, optical sheets should develop bends under the influence of heat in environments with very high ambient temperatures. However, such older illumination devices are not made so compact and lightweight as those of today, and this permits the above-mentioned clearance to be made sufficiently large to keep the problem negligible. By contrast, today, compact and lightweight illumination devices are keenly sought after, and accordingly the clearance in question is supposed to be made smaller and smaller in the future. This makes techniques for reducing heat-induced bends in optical and other sheets increasingly important in the future.
On the other hand, Japanese Patent Application Laid-Open No. H7-56166 teaches cutting off four corners of a light guide plate to prevent its deformation by the heat generated by a tubular light source. However, this invention pays no attention to the effect of heat on an optical or shielding sheet.
An object of the present invention is, in cases where an illumination device used in a liquid crystal display or the like has a sheet that exhibits high thermal expansion/contraction and that has a base prone to deformation due to thermal stress, to prevent wavelike bends that develop in the sheet under the influence of heat from being visually recognized in the display surface of the liquid crystal display or the like.
To achieve the above object, according to one aspect of the present invention, a sheet for an illumination device is provided inside the illumination device and fitted to a main body of the illumination device by being sandwiched, at least at a portion of the sheet and in the direction of the thickness of the sheet, between the main body and a frame fitted to the main body, and has a cut formed at least at one spot in the portion of the sheet in order to prevent bends that develop with a change in temperature.
According to another aspect of the present invention, an illumination device is provided with: a main body of the illumination device including a light source; a frame fitted to the main body; and a sheet or a plurality of sheets fitted to the main body by being sandwiched, at least at a portion of the sheet or sheets and in the direction of the thickness of the sheet or sheets, between the frame and the main body. Here, a cut is formed at least at one spot in the portion of at least one sheet among the sheet or sheets in order to prevent bends that develop with a change in temperature.
According to another aspect of the present invention, in a liquid crystal display including an illumination device and a liquid crystal display panel illuminated by the illumination device, the illumination device is provided with: a main body of the illumination device including a light source; a frame fitted to the main body; and a sheet or a plurality of sheets fitted to the main body by being sandwiched, at least at a portion of the sheet or sheets and in the direction of the thickness of the sheet or sheets, between the frame and the main body. Here, a cut is formed at least at one spot in the portion of at least one sheet among the sheet or sheets in order to prevent bends that develop with a change in temperature.